Plastic working causes crystalline grains of a polycrystalline metal material not to be oriented at random but to be statistically oriented in a specific orientation (preferred orientation) and develops the texture. The texture formed in a worked metal sheet by rolling is called rolling texture.
Another texture formed in the worked metal sheet is shear texture, which may be preferred over the rolling texture. As is known in the art, development of the shear texture improves the press formability (deep drawability) in aluminum alloy materials, the ductility in magnesium alloy materials, and the bend formability in copper alloy materials. Development of the shear texture also causes an easy direction of magnetization <001> to be orientated in parallel with a rolling direction in iron and steel materials.
The conventional rolling technique, however, introduces the shear texture only to the shallow surfaces of a resulting rolled metal sheet (hereafter referred to as ‘rolled sheet’) by the friction with rolls and does not succeed in sufficiently developing the shear texture into the sheet thickness of the rolled sheet. The conventional rolling technique accordingly does not exert the effects of the developed shear texture explained above.
A differential speed rolling technique of rotating a pair of an upper roll and a lower roll at mutually different speeds is adopted to introduce the shear deformation into the sheet throughout the thickness of the rolled sheet and develop the shear texture into the sheet throughout the thickness of the rolled sheet (see Non-Patent Document 1).
In order to reduce the rolling load, one proposed rolling technique rolls a metal sheet by differentiating between the lubricating oil quantities or the lubricating oil compositions of a liquid lubricant agent fed to an upper surface and to a lower surface of the metal sheet to make the friction coefficient of the metal sheet relative to an upper roll different from the friction coefficient of the metal sheet relative to a lower roll (see Patent Document 1). Namely this rolling technique rolls the metal sheet in the state of mutually differentiating between the lubricating oil quantities or the lubricating oil compositions fed to respective interfaces between the pair of rolls and the metal sheet to make the different friction coefficients on the respective interfaces.    Non-Patent Document 1: Tetsuo Sakai, Hiroshi Utsunomiya, and Yoshihiro Saito, ‘Introduction of Shear Strain to Aluminum Sheet and Control of Texture’, Kei-Kinzoku (Journal of Japan Institute of Light Metals), Japan Institute of Light Metals, November 2002, Vol. 52, No. 11, pp. 518 to 523    Patent Document 1: Japanese Patent Laid-Open No. Sho53-135861